Semiconductor processing improvements have increased the density and number of transistors and other devices that can be integrated together onto a single silicon integrated circuit die. These large systems-on-a-chip (SoC) often include some analog functions as well as many digital functions. However, the digital systems can be noisy as digital circuits switch between high and low states. Since analog circuits have a theoretically infinite number of states or voltage levels, while digital has just two states, analog systems are inherently more sensitive to noise.
Traditionally, analog systems were segregated from digital systems. However, with higher levels of integration and single-chip systems, analog systems must share a silicon substrate with noisy digital systems. Typically separate power supplies are used for digital and analog circuits on the same chip. The analog power supply, AVDD, is separate from the digital power supply VDD on the mixed-signal chip. The power-supply voltage levels for AVDD and VDD may be different, and AVDD is carefully designed to be as noise-free as possible.
A low-dropout (LDO) voltage regulator may be used to generate AVDD from VDD. Traditionally, LDO voltage regulators were external to the mixed-signal chip, allowing for better noise isolation. Also, the exact AVDD voltage level could be determined by a ratio of precision resistors. The precision resistors could be discrete external resistors, or could be trimmed during manufacture, such as by a laser trimming a resistor line, or by fuses. The voltage regulator could be on-chip, with the precision resistors off-chip.
Such trimming of resistors or blowing of fuses could be done on a larger SoC that includes the voltage regulator, but adding fuses to the process, or adding the step of laser trimming can significantly increase manufacturing costs and is thus undesirable. Once the resistance values are chosen, they may be irreversible as there may be no way to add back portions of a laser-trimmed resistor or to re-connect a blown fuse.
What is desired is a low-dropout voltage regulator that is integrated with analog and digital functions on a mixed-signal SoC. A LDO voltage regulator that has an adjustable output voltage by adjusting resistance values is desirable. An on-chip voltage regulator with software-programmable resistance values is desirable. Interpolation of adjacent resistance values is desirable to more precisely define resistance and output-voltage values. A high-precision user-programmable on-chip voltage regulator is desirable.